Certain alkenyl alkanoates, such as vinyl acetate (VA), are commodity chemicals in high demand in their monomer form. For example, VA is used to make polyvinyl acetate (PVAc), which is used commonly for adhesives, and accounts for a large portion of VA use. Other uses for VA included polyvinyl alcohol (PVOH), ethylene vinyl acetate (EVA), vinyl acetate ethylene (VAE), polyvinyl butyral (PVB), ethylene vinyl alcohol (EVOH), polyvinyl formal (PVF), and vinyl chloride-vinyl acetate copolymer. PVOH is typically used for textiles, films, adhesives, and photosensitive coatings. Films and wire and cable insulation often employ EVA in some proportion. Major applications for vinyl chloride-vinyl acetate copolymer include coatings, paints, and adhesives often employ VAE having VA in some proportion. VAE, which contains more than 50 percent VA, is primarily used as cement additives, paints, and adhesives. PVB is mainly used for under layer in laminated screens, coatings, and inks. EVOH is used for barrier films and engineering polymers. PVF is used for wire enamel and magnetic tape.
Because VA is the basis for so many commercially significant materials and products, the demand for VA is large, and VA production is frequently done on a relatively large scale, e.g. 50,000 metric tons or more per year. This large scale production means that significant economies of scale are possible and relatively subtle changes in the process, process conditions or catalyst characteristics can have a significant economic impact on the cost of the production of VA.
Many techniques have been reported for the production of alkenyl alkanoates. For example, in making VA, a widely used technique includes a catalyzed gas phase reaction of ethylene with acetic acid and oxygen, as seen in the following reaction:C2H4+CH3COOH+0.5O2→CH3COOCH═CH2+H2OSeveral side reactions may take place, including, such as, the formation of CO2. The results of this reaction are discussed in terms of the space-time yield (STY) of the reaction system, where the STY is the grams of VA produced per liter of catalyst per hour of reaction time (g/l*h).
The composition of the starting material feed can be varied within wide limits. Typically, the starting material feed includes 30-70% ethylene, 10-30% acetic acid and 4-16% oxygen. The feed may also include inert materials such as CO2, nitrogen, methane, ethane, propane, argon and/or helium. The primary restriction on feed composition is the oxygen level in the effluent stream exiting the reactor must be sufficiently low such that the stream is outside the flammability zone. The oxygen level in the effluent is affected by the oxygen level in the starting material stream, O2 conversion rate of the reaction and the amount of any inert material in the effluent.
The gas phase reaction has been carried out where a feed of the starting materials is passed over or through fixed bed reactors. Successful results have been obtained through the use of reaction temperatures in the range of −125° C. to 200° C., while reaction pressures of 1-15 atmospheres are typical.
While these systems have provided adequate yields, there continues to be a need for reduced production of by-products, higher rates of VA output, and lower energy use during production. One approach is to improve catalyst characteristics, particularly as to CO2 selectivity and/or activity of the catalyst. Another approach is to modify reaction conditions, such as the ratio of starting materials to each other, the O2 conversion of the reaction, the space velocity (SV) of the starting material feed, and operating temperatures and pressures.
The formation of CO2 is one aspect which may be reduced through the use of improved catalysts. The CO2 selectivity is the percentage of the ethylene converted that goes to CO2. Decreasing the CO2 selectivity permits a larger amount of VA per unit volume and unit time in existing plants, even retaining all other reaction conditions.
VA output of a particular reaction system is affected by several other factors including the activity of the catalyst, the ratio of starting materials to each other, the O2 conversion of the reaction, the space velocity (SV) of the starting material feed, and operating temperatures and pressures. All these factors cooperate to determine the space-time yield (STY) of the reaction system, where the STY is discussed in terms of grams of VA produced per liter of catalyst per hour of reaction time or g/l*h.
Generally, activity is a significant factor in determining the STY, but other factors may still have a significant impact on the STY. Typically, the higher the activity of a catalyst, the higher the STY the catalyst is able to produce.
The O2 conversion is a measure of how much oxygen reacts in the presence of the catalyst. The O2 conversion rate is temperature dependent such that the conversion rate generally climbs with the reaction temperature. However, the amount of CO2 produced also increases along with the O2 conversion. Thus, the O2 conversion rate is selected to give the desired VA output balanced against the amount of CO2 produced. A catalyst with a higher activity means that the overall reaction temperature can be lowered while maintaining the same O2 conversion. Alternatively, a catalyst with a higher activity will give a higher O2 conversion rate at a given temperature and space velocity.
It is common that catalysts employ one or more catalytic components carried on a relatively inert support material. In the case of VA catalysts, the catalytic components are typically a mixture of metals that may be distributed uniformly throughout the support material (“all through-out catalysts”), just on the surface of the support material (“shell catalysts”), just below a shell of support material (“egg white catalysts”) or in the core of the support material (“egg yolk catalysts”).
Numerous different types of support materials have been suggested for use in VA catalyst including silica, cerium doped silica, alumina, titania, zirconia and oxide mixtures. But very little investigation of the differences between the support materials has been done. For the most part, only silica and alumina have actually been commercialized as support materials.
One useful combination of metals for VA catalysis is palladium and gold. Pd/Au catalysts provide adequate CO2 selectivity and activity, but there continues to be a need for improved catalysts given the economies of scale that are possible in the production of VA.
One process for making Pd/Au catalysts typically includes the steps of impregnating the support with aqueous solutions of water-soluble salts of palladium and gold; reacting the impregnated water-soluble salts with an appropriate alkaline compound e.g., sodium hydroxide, to precipitate (often called fixing) the metallic elements as water-insoluble compounds, e.g. the hydroxides; washing the fixed support material to remove un-fixed compounds and to otherwise cleanse the catalyst of any potential poisons, e.g. chloride; reducing the water insoluble compounds with a typical reductant such as hydrogen, ethylene or hydrazine, and adding an alkali metal compound such as potassium or sodium acetate.
Various modifications to this basic process have been suggested. For example, in U.S. Pat. No. 5,990,344, it is suggested that sintering of the palladium be undertaken after the reduction to its free metal form. In U.S. Pat. No. 6,022,823, it suggested that calcining the support in a non-reducing atmosphere after impregnation with both palladium and gold salts might be advantageous. In WO94/21374, it is suggested that after reduction and activation, but before its first use, the catalyst may be pretreated by successive heating in oxidizing, inert, and reducing atmospheres.
In U.S. Pat. No. 5,466,652, it is suggested that salts of palladium and gold that are hydroxyl-, halide- and barium-free and soluble in acetic acid may be useful to impregnate the support material. A similar suggestion is made in U.S. Pat. No. 4,902,823, i.e. use of halide- and sulfur-free salts and complexes of palladium soluble in unsubstituted carboxylic acids having two to ten carbons.
In U.S. Pat. No. 6,486,370, it suggested that a layered catalyst may be used in a dehydrogenation process where the inner layer support material differs from the outer layer support material. Similarly, U.S. Pat. No. 5,935,889 suggests that a layered catalyst may useful as acid catalysts. But neither suggests the use of layered catalysts in the production of alkenyl alkanoates.
Taken together, the inventors have recognized and addressed the need for continued improvements in the field of VA catalysts to provide improved VA production at lower costs.